The present invention relates to a linear machine which includes an electromechanical converter with a linear movable piston which is arranged in a tubular cylinder. The invention also includes the use of this machine. In this application, primarily the disclosure is about use of the linear machine as an electrical motor, but it can also be relevant to make use of it as generator for the production of electrical energy.
Known are different types of uses of such linear machines. Examples are compressors, vibrators, Stirling machines, motors, and generators.
Such a linear machine comprises the following main parts: a piston, a coil arrangement, and a casing. The piston is magnetized, for example, with the aid of permanent magnets, which can be called an “armature” or “rotor”.
The alternating current in the coil will result in setting up a varying magnetic field in the machine, and this field will interact with the magnetic field of the permanent magnets. The interaction causes an energy transmission between the electrical energy in the coil arrangement, and the mechanical energy in the form of linear movement of the piston inside the casing.
A piston bar can connect the piston in the machine with an outer working element, for example, in a piston compressor or in a linear Stirling machine.
It is necessary that the piston does not move further than the casing permits. That means that the piston has to be decelerated down to a speed equal to zero at each end of its linear movement. This can be achieved by control of the magnetic field. When the power of the electric motor becomes large, at the same time, the electrical losses become large. This will reduce the power efficiency.
For this reason, linear machines with a power output of about 0.5 kW and higher comprise resonance-effective arrangements, as for example, springs, which attempt to pull and/or push the piston to the center position. If the coil is not under voltage, the piston may be pulled out of the center position before it is released. The resonance-effective arrangements will be provided so the piston may oscillate around the balance position. The piston then oscillates with a frequency equal to the natural frequency.
It is also important to achieve an effective energy transmission. Therefore it is desirable that the frequency of the alternating voltage in the coil arrangement is approximately equal to the natural frequency of the piston. This will result in resonance. In the case of perfect resonance, the electrical force will always be effective in the same direction as the direction of the movement of the piston. Then the electrical force will always supply energy to the piston. If the electrical force is not in resonance it will decelerate the piston in parts of the stroke. Then the electrical force must have a greater absolute value for transmission of the same amount of energy to the piston. This means more electric current in the coils than necessary, resulting in greater loss.
The natural frequency of the piston is, among other things, determined by the mass of the piston. If the mass of the piston is stated here, this mass includes all the components which are being set in motion for energy transformation between electrical and mechanical energy, including permanent magnets, the frame of the permanent magnets, gaskets, piston bars and external pistons in compressors and Stirling machines. Other factors which likewise have an influence on the natural frequency are the properties of the resonance-effective arrangements mentioned above.
When the piston oscillates linearly in the machine, accordingly a transformation of kinetic energy will take place in the piston whenever it is in the center position, to potential energy stored in the resonance-effective arrangements when the piston is in one or the other of its extreme positions, and so back again.
With the expression “stroke” it is meant the movement of the piston from a center position to a first extreme position, and until the piston is in the center position again with movement towards the same extreme position.
The effect of a linear machine is proportional to the electrical force, multiplied with the length of stroke and the frequency. In the case of known machines, the length of stroke and the frequency will often be limited, for example, due to a limited speed in view of the slide bearings. Today's focus is directed towards the use of a relatively lightweight piston which is guided with great force in the casing. The reason that the piston is required to be light-weight is that a heavy piston requires extremely strong springs. Expressed in another way: If the piston is heavy, much kinetic energy must be stored in the resonance-effective arrangement. This is what limits the effect in case of known linear machines until about 1 kW.
U.S. Pat. No. shows 6,379,125 shows an example of a linear compressor driven by a a linear motor. This motor comprises disk-shaped or helical springs to bring the piston to a neutral point. This technique cannot be applied to the manufacture of linear machines with a power output than about 1 kW.
GB-patent application 2 017 422 shows an example of a linear vibrator. Here the aim is to obtain a vibrator which has to perform relatively long strokes with a piston of relatively low mass. In this case, metal springs, and/or gas springs, are used as a resonance-effective arrangement. It is alleged here that the piston shall have low mass, with that, this technique can neither be used to manufacture more powerful linear machines. The reason is that a lightweight piston limits the weight of the magnets or coils in the movable part, and it is this maximum force, which shall transmit energy to the piston. A lightweight piston will also result in that low energy remains to be stored in the system. The energy outlet of the load will then result in that the system becomes overdamped, something that is exceptionally demanding in view of the controlling of the force on the piston.
From U.S. Pat. No. 4,067,667 (White 1978) a compressor is known with an oscillating motor with a free linear movable electric armature, which is integrated with two compressor pistons and a double gas spring. The gas spring comprises a circular piston in a circular cylinder at each end of the armature. First of all, this solution is complicated to construct and to maintain. Furthermore, it will have a very short stroke and also a small effective piston area. This creates great limitations relating to the mass of the armature and with that the power which can be transmitted. This construction limits the possibility of scaling up. This means that known machines are unsuitable for tasks which require more than a limited power or output.